1. Field of the Invention
The present invention relates to a wireless sensor-actor network (WSAN), and particularly to a least movement WSAN topology repair method.
2. Description of the Related Art
Wireless Sensor-Actor Networks (WSANs) have attracted much interest in recent years. WSANs can increase the effectiveness of numerous applications, such as homeland security, battlefield reconnaissance, space exploration, search and rescue, etc. A typical WSAN consists of a larger set of miniaturized sensor nodes reporting their data to significantly fewer actor (actuator) nodes. Sensors probe their surroundings and report their findings to one or multiple actors, which process the collected sensor reports and respond to emerging events of interest. An actor's response would depend on its capabilities, which varies based on the application and the expected role the actor plays. For example, an actor can deactivate a landmine, extinguish a fire and rescue a trapped survivor. It is worth noting that a heterogeneous set of actors may be employed and assigned complementary roles.
In most application setups, actors need to coordinate with each other in order to share and process the sensors' data, plan an optimal response and pick the most appropriate subset of actors for executing such a plan. For example in forest monitoring applications, such actors as fire trucks and flying aircraft need to collaborate with each other in order to effectively control a fire and prevent it from spreading. The selection of actors that need to be engaged can be based on many factors, such as the actor's capabilities, the actor's proximity to the detected event, and the actor's current load. All of these factors would require a frequent update of the actor's state. To enable such interactions, actors need to stay reachable to each other. In other words, a connected inter-actor network has to be maintained at all times.
An actor's failure can cause the loss of multiple inter-actor communication links and may partition the network if alternate paths among the affected actors are not available. Such a scenario will hinder the actors' collaboration, and thus have very negative consequences on the WSANs application. Therefore, the actors should be able to detect and recover from the failure of one of them. Given that the WSAN usually operates autonomously and unattended, the recovery should be a self-healing process for the network and should be performed in a distributed manner. In addition, the network recovery should be both quick and lightweight. Rapid recovery is desirable in order to maintain the WSAN responsiveness to detected events. In addition, the overhead should be minimized in order to ensure the availability of actors' resources for application-level missions.
However, actors are responsible for responding to the specific events and carry out tasks that must be consistent with the application goals. Therefore, unconstrained movement of the actor(s) with the goal of achieving efficiency in terms of reduced overhead can cause a serious failure at application level. In other words, an application unaware of recovery of the inter-actor connectivity can be impractical in many scenarios. For example, consider the following scenario where an application unaware recovery of the inter-actor connectivity can lead to a disastrous situation.
Life support medical units are unmanned robotic vehicles that are equipped with the necessary life support equipment, such as oxygen tanks and masks. These actor units are deployed in an area that got hit by a natural disaster, such as earthquake, hurricane, etc. Human body heat sensors are also deployed all over the area. The job of these sensors is to probe the existence of a live human being in the vicinity and report it to the actors. After receiving such a report, close-by actors are responsible to reach the location and provide necessary life support until the rescue team arrives. At the time when a unit (actor) is busy in providing emergency help to a survivor under the rubbles, task termination and the mobility of this unit may cause serious damage to the operation. However, after completing the operation, the unit can be mobilized to any location without constraints. Thus, a recovery mechanism is needed to determine the best connectivity restoration scheme under application-level task termination constraints. On the other hand, most of the recently proposed schemes found in the literature require every node to maintain partial knowledge of the network state. To avoid the excessive state-update overhead and to expedite the connectivity restoration process, these schemes rely on maintaining 1- or 2-hop neighbor lists and predetermined criteria for a node's involvement in the recovery. Nonetheless, 1-hop based schemes often impose high node repositioning overhead, and the repaired inter-actor topology using 2-hop schemes may differ significantly from its pre-failure status.
However, some WSAN applications require timely coordination among the actors. For example, during a combat operation, timely interaction among actors would be required in order to accurately track and attack a fast moving target. Thus, extending the shortest path between two actors as a side effect of the recovery process would not be acceptable. Therefore, a novel network restoration scheme is required.
In some mission critical applications, node movement is not much appreciated, and moving many actor nodes as a side effect of the recovery process could lead to an application mission failure. For example, moving away a number of actor nodes while busy extinguishing a fire or life supporting natural disaster victims could lead to a disaster. Hence, a recovery algorithm is needed that strives to relocate the least number of nodes and reduce the total travel distance and communication overhead. Consequently, there remains the problem of how to reconfigure WSAN when a particular node or set of nodes fails.
Thus, a least movement WSAN topology repair method solving the aforementioned problems is desired.